Thymic Neuroendocrine Cell Tumor with Blood‑Filled Caverns

Three cases of primary thymic neuroendocrine tumors resembling a vascular neoplasm were reported as “Angiomatoid neuroendocrine carcinoma of the thymus.” Recently, we encountered another case of a thymic neuroendocrine cell tumor that grossly mimicked a vascular neoplasm. A man in his early 60 s, who presented with right thoracic pain, was admitted to our hospital. He had a 45 × 35 mm vascular-rich tumor in the thymus and underwent total thymectomy. Histopathological examination revealed that the tumor was composed of many blood‑filled caverns lined with stratified conventional neuroendocrine tumor cells expressing insulinoma-associated protein 1 and synaptophysin immunoreactivity. Notably, the blood-filled caverns were not lined with CD31-positive endothelial cells, as previously reported. By contrast, the caverns were focally lined with cells expressing SRY-Box Transcription Factor 17, which is well characterized to drive the conversion of fibroblast progenitor cells to endothelial cells by its transcriptional property. However, SRY-Box Transcription Factor 17 immunoreactivity was not restricted to the nucleus in blood-filled caverns and was also detected in the nucleus of endothelial cells in tumor vessels in the canonical carcinoid area. Dismaturation of endothelial cells might participate in the angiomatoid features of thymic neuroendocrine cell tumors.